Materials made up of papermaking sheets are already known, such as for example those described in U.S. Pat. No. 4,645,565 and which, being constituted of a fiber-reinforced thermoplastics matrix, are destined to be transformed by molding-stamping into parts of complex shape.
The advantage of these materials, over other products obtained by conventional processes of the plastics industry, resides in the fact that the reinforcing fibers, being for example, glassfibers, are, during the aqueous dispersion process, all individualized and evenly distributed in the three dimensions.
Such materials however are still found to have certain limitations compared with the ideal material sought by the man skilled in the art:
1. Their conversion into a finished piece, i.e. the step following the production on a papermaking machine implies first of all, a pre-heating at a temperature higher than the melting point of the thermoplastic material. Such pre-heating can be carried out in two ways:
either directly, on the necessary quantity of papermaking sheets placed between two heated plates; PA0 or indirectly, with plates obtained by hot-densifying of the above papermaking sheets, said plates being in turn pre-heated by infra-red radiation or hot air ovens.
In both cases, and in particular in the second case, the balance energy is unsatisfactory: in particular, the fact that at the end of the drying, the papermaking sheets have already reached a temperature higher than 100.degree. C., is not exploited. In addition, cutting of the sheets and plates is necessary.
Finally, the pre-heating in an oven imposes the addition of a much higher proportion of thermal stabilizers than in the conventional processes of injection molding.
2. When pre-heated, the quantity of material to be molded is transferred into a colder mold where it is cooled under pressure.
If, as if often the case, and the object with such materials, the molded piece is complex, namely that it comprises ribs, perpendicular planes, whereas it is known to anyone skilled in the art that in order to fill up the mold, the quantity of hot material must be deposited in that mold in the form of a stack of sheets or of plates, the base of the stack having a surface smaller than the mold projected surface, this is in order that during closure of the mold, the hot material flows in all the directions, over all the planes and in the smallest recesses.
However, to obtain such a flowing out of the non-cooled material requires the application of pressures of 100 to 200 bars, so that when the target pieces are large sized, hydraulic presses of several thousands of tons have to be used.
3. Another disadvantage caused by such spreading out of the hot material is that although, contrary to other plastics processes, the average mechanical properties are constant in every part of the finished piece, they remain variable according to the three dimensions since the reinforcing fibers are re-oriented during the spreading.
4. Another limitation, also due to the material flowing during molding, is that, in applications where the fibrous reinforcement could be partly constituted of low-priced cellulose fibers instead of synthetic fibers such as glassfibers, said cellulosic fibers cannot be used because, with their high specific surface area, they would considerably increase the viscosity of the pre-heated material and thus annihilate its flowing capacity.
5. Another problem common to all the compression molding techniques is that certain pieces must have void spaces and that such spaces can only be formed after molding, hence by an additional cutting operation involving loss of material.
6. Finally, with certain applications, it is necessary that one of the faces of the final piece has a special surface finish. To this effect, a surface sheet such as described in U.S. patent application Ser. No. 4,451,539 can be used, but this technique requires, on the one hand, a sheet of larger surface than the laid out surface of the final piece, which involves a loss of material, and on the other hand, and in parallel, a second pre-heating station for that surface sheet.